With recent progress in the semiconductor technology and packaging engineering, more portable personal communication systems are made available, as the dimensions of these circuits become smaller and the transmission lines are being formed with shorter distances from each other.
In frequency conversion systems, such as millimeter-wave radios, for example, discrete semiconductor components with low integration levels are assembled. These systems are often assembled using expensive and bulky waveguides and package-level or board-level microstrip structures to interconnect semiconductors.
Power couplers are used widely in frequency conversion systems such as the millimeter radios to divide power in an input path into two or more paths. In these millimeter radios, the power couplers used may include microstrip branch lines of different lengths to generate on-chip oscillator signals of different phases. For the purpose of shifting the phase by 90°, for example, quarter wave-length transmission lines may be used. As the dimension of the power coupler is reduced, the distances between the branch lines are also reduced. A known power coupler is the branch-line directional coupler.
Shown in FIG. 1 is an exemplary branch-line directional coupler 14 with four branch lines 10, 12. As shown in FIG. 1, the branch lines 10, 12 are a quarter-wavelength long at the center design frequency, connected at connecting ports 1-4. Branch lines 10 connecting ports 1 and 2, and ports 3 and 4, have a characteristic impedance of Zo/1.414. Branch lines 12 connecting ports 2 and 3, and ports 1 and 4, have a characteristic impedance of Zo. When port 1 is driven by a signal, the voltage at port 3 lags the voltage at port 2 by 90°, and the voltage at port 4 is at a null (i.e. no output).